Metropolitan Transportation Authority (MTA) Electrical Practice Test

A rheostat is a device used to control current by changing resistance in a circuit; it is a variable resistor. By turning a knob or sliding a contact along a resistive element, you alter the length of the path the current must travel, which changes the resistance seen by the circuit. According to Ohm’s law (I = V/R), increasing the resistance lowers the current, while decreasing the resistance raises it. This makes rheostats handy for adjusting things like lamp brightness or motor speed where a controllable current is needed.

It isn’t a fixed resistor, which would keep the same resistance and current, nor a transformer, which changes voltage and current without altering resistance in the same way. It also isn’t a capacitor, which stores and releases energy rather than controlling steady current through resistance.